[testing] Add more assemblies to depend on during linking of library tests

Author: Mitchell Hwang

eng/testing/ILLink.Descriptor.xunit.xml

@@ -9,7 +9,9 @@
     </type>
     <type fullname="Xunit.Sdk.FactDiscoverer" />
   </assembly>
-  <assembly fullname="xunit.core">
-    <namespace fullname="Xunit" />
+  <assembly fullname="xunit.core"/>
+  <assembly fullname="Microsoft.DotNet.XUnitExtensions"/>
+  <assembly fullname="TestUtilities">
+    <namespace fullname="System" />
   </assembly>
 </linker>